by Jennifer Jacobs and Cat Camia, USA TODAY

His retirement is expected to leave a huge void for Iowa, but clears the way for Democrats trying determine their lineup for the mid-term elections.

The next generation of Democratic candidates depended on whether Harkin, 73, chose to run again, party operatives have said. Rep. Bruce Braley is the favored replacement, Democrats have said.

"I have mixed feelings," said Harkin, who was born in Cumming and has served in the Senate for 27 years and in the U.S. House 10 years before that. "You know what, it's somebody else's turn."

He added: "To walk away from this position and this power is not an easy thing. But I think it's the right thing. And I have two years left. I'm not passing the torch sitting down; it's a running relay. I intend to be very active over the next two years."

The news was unexpected: Many leading Democrats thought Harkin would run again. He and his Republican counterpart, Sen. Chuck Grassley, 78, are a major force in the federal government.

Following Harkin's announcement, Senate Majority Leader Harry Reid of Nevada called the Iowan a "pillar of the Senate" whose "work on behalf of people with disabilities has changed millions of lives and advanced the cause of equality for which he has fought tirelessly for decades."

Sen. Charles E. Schumer of New York, the No. 3 Democrat, said in a statement: "Tom Harkin comes from the heartland of America and represents the best of that region's values, combining strong convictions with a deep, personal humility.

"His hard work and abiding commitment to caring for one's neighbor has made him one of the Senate's most respected progressives," Schumer said. "He will be deeply missed by senators on both sides of the aisle."

Harkin is popular in Iowa, regularly earning job approval ratings in the 60% range in the Iowa Poll.

He is a big voice on social issues, and he has the ear of party leaders. Since Democrats control his chamber, he can move bills and tuck in an amendment here or there. He has left a thumbprint on agriculture, improvements for people with disabilities, health care and various progressive causes for decades.

Harkin shoulders an important assignment in the Senate: He's chairman of the HELP committee, which stands for Health, Education, Labor and Pensions, a huge jurisdiction. The No Child Left Behind education law and the 2010 health care reform law - the Affordable Care Act, often called Obamacare - originated there. Those assigned to this powerful committee generally aren't supposed to serve on other committees, but Harkin has taken on three others: agriculture, appropriations and small business.

Democrats have to defend 20 seats in 2014; the Republicans 14. Democrats currently have a 55-45 voting advantage, with two independents organizing with them.

Harkin is the third senator up for re-election to say he is not running again. His decision follows Friday's surprise announcement that Saxby Chambliss, R-Ga., would not seek re-election. Jay Rockefeller, D-W.Va., is also retiring.

Harkin's seat is likely to be in play in a swing state that Obama won both times.